This invention relates generally to attitude control systems for spacecraft and, more particularly, to attitude control systems using magnetic torque rods. As is well known, operational spacecraft fall freely in space and, without some mechanism of attitude control, would rapidly lose their desired orientation or attitude. One of the most common mechanisms for attitude control of spacecraft operating near the earth is a magnetic torquer. Basically, the torquer includes a magnetic dipole, which interacts with the earth""s magnetic field to produce a desired torque or turning moment. The torquer includes an iron core rod and a conductive winding having multiple turns about the rod. A source of current applied to the winding produces a dipole magnetic field of a desired strength, which interacts with the earth""s magnetic field to produce the desired torque on the rod. The rod, of course, is rigidly held in the spacecraft, and the torque is transferred to the entire spacecraft. Typically, three torque rods are used, to provide attitude control in three-dimensional space.
Because of the iron core rod in each magnetic torquer, the device is relatively heavy and contributes significantly to the launch cost, which can be up to $50,000 per pound for payloads launched into geosynchronous orbit. Therefore, it will be appreciated that there is an ongoing need to reduce the weight or increase the efficiency of magnetic torquers, because this will reduce the launch cost of each space mission in which torquers are employed.
The relationship between the electrical current applied to a magnetic torquer and the resultant dipole magnetic moment is also well known. The magnetic moment produced by the dipole, increases linearly as the applied current is increased either positively or negatively from zero. Outside of a linear regime extending symmetrically on each side of a zero current point, the magnetic moment increases less rapidly than the applied current and the characteristic curve of the device begins to flatten off. If the current is then reduced toward zero, the magnetic field strength generated by the device tends to retain its previously higher value and there may be a residual dipole moment when the current reaches zero again. The characteristic relationship between applied current (or voltage) to the dipole magnetic moment follows a well known hysteresis curve. For spacecraft torquers, it is desirable to have a near-zero residual magnetic moment when the current is reduced to zero, since any residual torque will affect spacecraft orientation over time and will require subsequent correction.
Spacecraft magnetic torquers are normally operated only in the linear regime, in which the dipole moment is directly proportional to the applied electrical current or voltage. Spacecraft attitude controllers are typically linear controllers, which expect sensors and actuators to have linear behavior. In most cases, the analysis to verify stability and performance of an attitude control system has been a linear analysis, and non-linear components would render this analysis invalid. Therefore, imposing a linearity requirement on magnetic torquers simplifies control of the spacecraft but has a penalty in that the maximum moment and torque obtainable from each torquer is limited by the magnetic moment at the end of the linear regime. Larger moments can be obtained only by using a larger and heavier torque rod. Clearly, it would be desirable to increase the linear range of the magnetic torquer, since this would increase the efficiency of the device and reduce the weight required to produce a given maximum magnetic dipole moment. The present invention is directed to this end.
The present invention provides a mechanism and a corresponding method for its use, for increasing the effective linear range of operation of a magnetic torquer for spacecraft attitude control, the torquer having a magnetic core and a conductive coil wound on the core. Briefly, and in general terms, the method comprises the steps of generating a corrected command signal needed to produce a desired magnetic moment, wherein the corrected command signal compensates for non-linearity in variation of the magnetic moment with the command signal; and applying the corrected command signal to the magnetic torquer, to produce the desired magnetic moment. The magnetic torquer is operated over a range extending beyond a linear regime in which torquers are normally operated, and a significantly higher torque is obtainable from a torquer of given weight.
In one disclosed embodiment of the invention, the step of determining a corrected command signal includes sensing magnetic field strength near the torquer, to obtain a signal indicative of magnetic moment; generating an error signal by computing the difference between a nominal command signal and the signal indicative of magnetic moment; and combining the nominal command signal with the error signal to obtain the corrected command signal. In accordance with an alternate embodiment of the invention, the step of generating a corrected command signal includes applying the desired magnetic moment to a mathematical model of the magnetic torquer; and obtaining from the mathematical model the corrected command signal corresponding to the desired magnetic moment. More specifically, the step of generating a corrected command signal in this embodiment includes retrieving the corrected command signal from a look-up table relating magnetic moments to corresponding command signals. The method may further include the step of generating data for storage in the look-up table.
The invention may also be defined as a magnetic torquer for use in spacecraft attitude control, comprising a magnetic core and an actuating coil wound around the core; an adjustable power supply for receiving a command signal and generating an actuating current to the coil; and means for generating a desired magnetic moment in the torquer, over an extended range that exceeds a conventionally used linear range, whereby a larger torque can be produced without increasing the mass of the magnetic core. In one embodiment of the invention, the means for generating a desired magnetic moment includes a feedback control system that generates a corrected command signal to provide the desired magnetic moment. More specifically, the feedback control system includes a magnetic field sensor to provide a signal indicative of the magnetic moment generated by the torquer; a subtractor circuit for generating an error signal from the difference between the signal from the magnetic field sensor and a nominal command signal; and a signal combiner circuit for combining the error signal with the nominal command signal, to obtain a corrected command signal to be applied to the adjustable power supply.
In the alternate embodiment of the invention, the means for generating a desired magnetic moment includes a mathematical model of the magnetic torquer; wherein the model provides for any given desired magnetic moment a corrected command signal for application to the adjustable power supply.
It will be appreciated from the foregoing that the present invention represents a significant advance in magnetic torquers for use in spacecraft. In particular, the invention extends the effective linear range of a torquer, to provide for more efficient operation and a higher maximum torque from a torquer of given weight. Other aspects and advantages of the invention will become apparent from the following more detailed description, taken in conjunction with accompanying drawings.